Understanding the biological mechanisms and biomarkers of psychiatric disease is critical for understanding, assessing risk, and designing treatments of disorders such as post traumatic stress disorder (PTSD). In this regard, it was recently reported that the neuropeptide PACAP and its plasma membrane receptor PAC1 are linked to PTSD at both genetic and epigenetic levels. These findings complement a considerable set of prior evidence implicating PACAP/PAC1 signaling in stress and fear circuitries. Experiments in the proposal will use gene targeting approaches to dissect at the cellular, molecular, and behavioral levels, the involvement of PACAP-PAC1 signaling in the circuitry regulating fear in mice. The results are hoped to lay a mechanistic foundation for the development of an entirely new set of therapeutic targets for PTSD based on PAC1 signaling and downstream actions, and may also lead to the discovery of novel and robust biomarkers associated with this pathway.